A surge suppression device is an electronic device used for prevent various electronic equipments, instruments, and communication circuits from damage from surge current or over-voltage caused by sudden interference external to electrical circuits.
A surge suppression device typically comprises one or more metal-oxide varistors (MOVs) connected in parallel between a service power line and a ground or neutral line, or between a neutral line and a ground line, for absorbing and dissipating the energy related to the over-voltage. MOVs are non-linear, electronic devices that are frequently subjected to various external stresses during operation, such as temperature stresses or transient voltage surge stresses.
When subject to over-voltage, i.e., voltage higher than its rating value, MOV degrades, causing increase of leakage current and in most cases overheating, and possibly thermal breakdown short circuit. The heating of MOV elevates the temperature of the surge suppression device containing the MOV. When the temperature reaches the ignition temperature of combustible materials surrounding the MOV, such as epoxy coatings or plastic housing, it may cause fire.
In order to reduce the risk of catching fire due to surge suppression devices, a thermal protector for MOV was proposed. The thermal protector of this kind is able to separate a failed MOV from a power supply circuit under certain circumstances, therefore to some extent preventing the surge suppression devices from catching fire. However, the disadvantages reside in that, if the MOV is already suffered from breakdown short circuit before the open of the connection point of the thermal protector, an electric arc will be generated between the gap as formed after the disconnection of the thermal protector. The arc current, in this situation, equals to the short-circuit current of the power supply system. An ordinary thermal protector is possibly not able to distinguish such an arc. In another aspect, even though the MOV is not suffered from breakdown short circuit before the open of the connection point of the thermal protector, an electric arc is still possibly generated due to existence of relatively high voltage and/or relatively small gap distance between the electrode of the thermal protector contacting the MOV and the electrode on the MOV surface. Therefore, the fault current originating from the power supply system may be maintained and the risk that the surge suppression device may catch fire still exists.
Therefore, some surge suppression devices are incorporated with an arc extinguishing mechanism, which overcome the disadvantage that conventional surge suppression devices having a thermal protector can only block small fault current. For example, U.S. Reissue Pat. No. RE42,319E to MERSEN France SB SAS disclosed a circuit protection device which is shown in FIG. 1 of the present invention, wherein the device comprises an arc shield 188 movable along slotted rails 138. One end of the arch shield 188 is normally contact with a compression spring 139 and the other end is contact with the arm portion 152c of the contact element 152. The arm portion 152c prevents the arc shield 188 from moving upward. When the solder material 182 starts to melt due to the overheat of the MOV 112, the arm portion 152c will separate with the MOV 112, whereby the arc shield 188 is released and moved along the rail 138 and finally stops at a position between the MOV 112 and the arm portion 152, so as to shield any electrical arc that may be generated.
However, the device of RE42,319E suffers from a disadvantage that the arc shield 188 applies a force, in a direction substantially perpendicular to the movement direction thereof, to the arm portion 152c due to the existence of the spring 139, such that the arm portion 152 is stressed to separate from the solder material 182 and the MOV 112. This will cause the soldering strength of the thermal switch to decrease over time and the contact element is prone to separate from the MOV in nature or when subject to electromagnetic force generated due to electrical surge, causing failure of the device. If the thermal switch is disconnected while occurrence of electrical surge, a high voltage may generated and applied to the parallel equipments to be protected, causing risk of damage of the latter.